This invention relates to a noise suppression device for audio equipment, and more particularly to a noise suppression device provided between an audio source and an amplifier section or between a pre-amplifier and a main amplifier to eliminate a noise component in an automobile stereo set.
In the component car stereo, an audio source A, such as a cassette tape deck or an FM receiver, is coupled to an amplifier section B as shown in FIG. 1 by means of shielding wires. In the case where an impedance of the grounded wire l' is not neglegible, various noises emanating from automobiles, such as switching noises, ignition noises, horn noises or the like, will be applied to the grounded wire l', thereby causing a noise current to flow therethrough. For this reason, the noise voltage e.sub.n equivalently coupled in series to the audio source output signal e.sub.i will be applied between the output terminal E.sub.out of the audio source A and the input terminal E.sub.in of the amplifier section B, it is then amplified by the amplifier section and emitted from a loud speaker (not shown in FIG. 1) coupled to the output of the amplifier section B. In addition, since a part of the power source current of the amplifier section A flows in the grounded path, the ripple voltage thereof will be superimposed on the signal flowing between the output E.sub.out of the audio source A and the input terminal E.sub.in of the amplifier section B. This creates unwanted distortion.
In order to eliminate the aforementioned of these problems it has been conventionally proposed to interpose a line transformer or photo-coupler between the output terminal of the audio source A and the input terminal of the amplifier section B or in a multi-amplifier system between the output of a pre-amplifier and the input of a main amplifier. However, the line transformer technique is disadvantageous in that it has a large physical configuration and is relatively heavy. Besides, it is expensive if the device having a good treble frequency characteristic is chosen. In the photo-coupler, because the linearity of electrical properties is inferior, it is disadvantageous in that fidelity is degraded. In addition, the photo-coupler is also expensive. In the multi-amplifier system, a number of the line transformers or the photo-couplers must be provided, the overall audio equipment will accordingly become expensive.